The present invention relates, in general, to a passenger detection system for vehicles such as buses, and more particularly to an acoustic system for detecting ingress and egress of passengers whereby an accurate running count of the number of passengers entering and leaving the vehicle can be maintained.
Numerous methods for counting passengers and for automatically keeping a track of the number of passengers on board a passenger vehicle such as a bus have been devised over the years, but for a variety of reasons none have been entirely satisfactory. Turnstiles at the entry and exit doors are unacceptable primarily because of the congestion they produce when there is a large number of passengers or when passengers are carrying parcels. The use of tickets to keep track of passengers has been proposed, but this procedure is time consuming and confusing, and this is unsatisfactory. Seat switches are inaccurate, for they record heavy packages as passengers and cannot count standess, while treadle counters provide inaccruate readings when anomalous traffic situations occur, or when two passengers are on the same step. Photoelectric counters of the beam-breaking variety have been tried, but the beams cannot discriminate between passengers and packages, and cannot distinguish two-way traffic. Moreover, a passenger standing on a step and swaying with the motion of the vehicle could cause multiple readings in a photoelectric device.
Thus, there is a need for a system that can reliably and accurately count passengers, can discriminate between those entering and exiting, is insensitive to packages or carried articles such as canes and umbrellas, and which overcomes the various problems encountered with prior art systems. In accordance with the present invention, these prior art difficulties are best overcome by techniques that sense motion through a volume of space that is bounded in three dimensions. It has been found that a range-filtered pulse-doppler sonar system can be designed to ignore all targets except those moving through a particular volume of space in a particular direction and that such a system can provide an economical and effective solution.